1. Field of Invention
The present invention relates to a pump that moves a fluid by changing the volume of the inside of a pump chamber using, for example, a piston or a diaphragm.
2. Description of Related Art
A conventional example of such a type of pump typically has a structure that is similar to the structure disclosed in Japanese Unexamined Patent Application Publication No. 10-220357 including a check valve that is mounted between an entrance passage and an exit passage, and a pump chamber defining a volume can be changed.
An example of a structure of a pump that produces a flow in one direction by making use of the viscosity resistance of a fluid is disclosed in Japanese Unexamined Patent Application Publication No. 8-312537. In this Publication, a valve is provided at an exit passage, and the fluid resistance at an entrance passage is greater than the fluid resistance at the exit passage when opening the valve.
An example of a structure of a pump that is made to be more reliable without using a movable part at a valve is disclosed in Published Japanese Translations of PCT International Publication for Patent Application No. 8-506874. This Publication discloses a compression structural member in which an entrance passage and an exit passage have shapes that are formed so that the pressure drops differ depending on the direction of flow.
However, in the structure disclosed in Japanese Unexamined Patent Application Publication No. 10-220357, both the entrance passage and the exit passage require a check valve, so that there is a problem in that pressure loss is high when a fluid passes through the two check valves. In addition, since fatigue damage may occur due to repeated opening and closing of the check valves, there is another problem in that the larger the number of check valves used, the lower the reliability of the pump.
In the structure disclosed in Japanese Unexamined Patent Application Publication No. 8-312537, in order to reduce back flow that is produced in the entrance passage during a pump discharge stroke, it is necessary to make the fluid resistance at the entrance passage to be large. When the fluid resistance is made to be large, fluid enters a pump chamber against the fluid resistance during a pump suction stroke, so that the suction stroke takes longer than the discharge stroke. Therefore, the frequency of the discharge-suction cycle of the pump becomes considerably low.
A small, light, high-output pump can be formed by an actuation operation at a high frequency using a piezoelectric element as an actuator for moving a piston or a diaphragm in up and down directions. The piezoelectric element is such that the displacement is small during one period but the response frequency is high, and has the characteristic of providing higher output energy the higher the frequency at which the actuation operation is performed up to the time of resonant frequency of the element. However, in the structure disclosed in Japanese Unexamined Patent Application Publication No. 8-312537, as mentioned above, an actuation operation can only be performed at a low frequency, so that there is a problem in that a pump that makes full use of the features of the piezoelectric element cannot be realized.
In the structure disclosed in Published Japanese Translations of PCT International Publication for Patent Application No. 8-506874, in accordance with an increase or a decrease in the volume of the pump chamber, the net quantity of flow is caused to be in one direction due to differences in pressure drops depending on the direction of flow of the fluid that passes through the compression structural member. Therefore, the back flow rate increases as external pressure (load pressure) at the exit side of the pump increases, resulting in the problem that the pump no longer operates at high load pressure. According to the treatise entitled xe2x80x9cAn Improved Valve-less Pump Fabricated Using Deep Reactive Ion Etchingxe2x80x9d presented in 1996 IEEE 9th International Workshop on Micro Electro Mechanical Systems, the maximum load pressure is of the order of 0.76 atmospheres.
Accordingly, it is an object of the present invention to provide a small, light and high-output pump which can operate under high load pressure, which makes it possible to reduce pressure loss and to increase its reliability by decreasing the number of mechanical on-off valves used, and which makes full use of the features of a piezoelectric element when the piezoelectric element is used as an actuator that actuates a piston or a diaphragm as a result of reducing the period of increasing and decreasing the volume of a pump chamber.
In order to overcome the above-described problems, according to a first aspect of the invention, a pump is provided that includes a pump chamber whose volume is changeable by a member including a piston and a diaphragm, an entrance passage used to make working fluid flow into the pump chamber, and an exit passage used to make the working fluid flow out from the pump chamber. A combined inertance value of the entrance passage is smaller than a combined inertance value of the exit passage. The entrance passage is provided with a fluid resistance member in which fluid resistance when the working fluid flows into the pump chamber is smaller than fluid resistance when the working fluid flows out of the pump chamber.
An inertance value L is determined by the expression L=xcfx811/S, wherein the cross-sectional area of a flow path is S, the length of a flow path is 1, and the density of the working fluid is xcfx81. When a passage pressure difference is P, and the flow rate in a passage is Q, and when the inertance L is used to transform the formula of the movement of a fluid inside a passage, the relationship P=Lxc3x97dQ/dt is derived. In other words, the inertance value indicates the degree of influence that unit pressure has on the change in the flow rate per second. The larger the inertance value, the smaller the change in the flow rate per second, whereas the smaller the inertance value, the larger the change in the flow rate per second.
The combined inertance value for parallel connection of a plurality of passages and for series connection of a plurality of passages having different shapes is calculated by combining the inertance values of the individual passages similarly to the way the inductance values for parallel connection and those for series connection in electrical circuits are combined.
Here, the entrance passage refers to a passage that extends from the inside of the pump chamber to a fluid flow-in-side end surface of an entrance connecting tube that connects the pump to the outside. However, when a pulsation absorbing device, such as that described below, is connected, the entrance passage refers to a passage that extends from the inside of the pump chamber to a connection portion with the pulsation absorbing device. Further, when the entrance passages of a plurality of pumps merge as described below, it refers to a passage from the inside of the pump chamber to the merging portion.
In accordance with the operation of the pump having the structure such as that described above with regard to the first aspect of the invention, when the piston or the diaphragm operates in the direction in which the volume of the pump chamber becomes small, this direction is, at the entrance passage, the direction in which the fluid flows out, so that the fluid resistance of the fluid resistance member is large, thereby making the fluid flowing out from the entrance passage very small or zero. On the other hand, at the exit passage, when the pressure inside the pump chamber increases in accordance with the shrinkage ratio of the fluid, the flow rate in the direction in which the fluid flows out from the pump chamber increases in accordance with the difference between the pressure inside the chamber and the load pressure and the inertance value.
When the piston or the diaphragm operates in the direction in which the volume of the pump chamber increases, the pressure inside the pump chamber decreases. When the pressure inside the pump chamber becomes less than the external pressure of the entrance passage, this direction is, at the entrance passage, the direction in which the fluid flows in, so that the fluid resistance of the fluid resistance member becomes small, thereby causing an increase in the flow rate in the direction in which the fluid flows into the pump chamber in accordance with the pressure difference and the inertance value of the entrance passage. On the other hand, in the exit passage, in accordance with the difference between the load pressure and the pressure inside the pump chamber, and the inertance value, the flow rate in the direction in which the fluid flows out from the pump chamber is reduced.
Here, at the entrance passage, the greater the rate of increase of the flow rate of the fluid that flows in, fluid of an amount corresponding to the volume that has flown out from the inside of the pump chamber can be made to flow into the pump chamber while the amount of decrease in the flow rate of the fluid that flows out at the exit passage is small. Therefore, in the present invention, the combined inertance value of the entrance passage is made to be smaller than the combined inertance value of the exit passage.
When this is performed, the number of mechanical on/off valves is reduced, thereby reducing pressure loss and making the pump more reliable. In addition, as described below, since the time required to increase the volume of the pump chamber and the time required to reduce it can be of the same order, an actuator that actuates the piston or the diaphragm can be made to operate at a high frequency. Therefore, when a piezoelectric element is used for the actuator, it is possible to realize a small, light and high-output pump that makes full use of the features of the piezoelectric element.
According to a second aspect of the invention, in the pump of the first aspect, a pulsation absorbing device that absorbs pulsation of the working fluid is connected to a working fluid entrance side of the entrance passage. When this is performed, since pressure pulsation caused by the opening and closing of the check valve is restricted, it is possible to restrict the influences of the inertance value of the entrance connecting tube and that caused by an external pipe connected to the entrance connecting tube. In correspondence with the amount by which the influences of the inertance value of the passage inside the entrance connecting tube is restricted, a volume of a flow that is equal to a volume of a flow that has flown out from the exit can flow into the pump chamber in a short time period by the pump of the first embodiment. Therefore, it is possible to cause the period in which the volume of the pump chamber is increased and decreased to be smaller, thereby making it possible to realize a pump that makes full use of the features of a piezoelectric element used as an actuator that actuates a piston or a diaphragm. Further, it is possible to connect a pipe of a freely chosen dimension to the pump without degrading the performance of the pump.
According to a third aspect of the invention, in the pump of the first or second aspects, a plurality of the pump chambers are provided, the entrance passage used to make working fluid flow into the plurality of pump chambers merges at the working fluid entrance side, and the pump further includes a driving device that performs a driving operation by shifting a timing at which volumes of the plurality of pump chambers are changed. By forming this structure, pressure pulsation produced by a change in the fluid resistance of the fluid resistance member is restricted at the entrance connecting tube, disposed upstream from the merging portion, to connect the pump to the outside and at an external pipe portion connected to the entrance connecting tube. Therefore, advantages that are similar to those provided by the structure of the second aspect are provided.
In particular, it is preferable that three pumps be used, and a driving operation be performed by shifting a timing at which the volume of a chamber of each pump is changed by ⅓ period, because the restriction effect is large in contrast with the small number of parts used. It is preferable that this feature be combined with that of the second aspect because the effect of restricting pressure pulsation becomes even greater.
According to a fourth aspect of the invention, in the pump of the third aspect, the exit passage used to make the working fluid flow out from the plurality of pump chambers merges at a working fluid exit side.
When this structure is formed, pressure pulsation produced by a change in the volume of each pump chamber is restricted at an exit connecting tube, disposed downstream from the merging portion, to connect the pump to the outside and at an external pipe portion connected to the exit connecting tube. Therefore, it is possible to connect a pipe of a freely chosen dimension to the exit side of the pump.
According to a fifth aspect of the invention, in the pump recited in any of the first to fourth aspects, a pulsation absorbing device that absorbs pulsation of the working fluid is connected to the working fluid exit side of the exit passage.
When this structure is formed, pressure pulsation produced by a change in the volume of the/each pump chamber is restricted at the exit connecting tube, disposed downstream from the merging portion, to connect the pump to the outside and at an external pipe portion connected to the exit connecting tube. It is preferable to combine this feature with that of the fourth aspect because the effect of restricting pressure pulsation becomes even greater. Therefore, it is possible to connect a pipe of a freely chosen dimension to the exit side of the pump.
According to a sixth aspect of the invention, in the pump of any of the first to fifth aspects, the fluid resistance member is a check valve. Examples of fluid resistance members include those that make use of the nature of a fluid, such as those that are only formed by electrodes and that use working fluid as electroviscous fluid (a fluid whose viscosity increases when a voltage is applied) and a compression structural member disclosed in Published Japanese Translations of PCT International Publication for Patent Application No. 8-506874. However, these fluid resistance members are not very effective in preventing a fluid inside a pump chamber from flowing out to the outside through an entrance passage when the pressure inside the pump chamber becomes high. In other words, these fluid resistance members do not have much checking effect. Therefore, it is preferable to use a check valve that prevents back flow as the fluid resistance member.
When this structure is formed, the piston or the diaphragm operates in a direction in which the volume of the pump chamber/each pump chamber becomes small, so that back flow at the entrance passage when the pressure inside the pump chamber/each pump chamber becomes high is prevented from being produced. Therefore, it is possible to sufficiently increase the pressure inside the pump chamber/each pump chamber, so that, even when the load pressure is high, the working fluid can be sent towards the load side. In addition, the load pressure can be maintained when the pump is stopped.
According to a seventh aspect of the invention, in the pump of the second to fifth aspects, the pulsation absorbing device includes a resilient wall chamber which has at least a portion thereof formed by a resilient wall, and whose amount of change in volume per unit pressure is greater than the working fluid. When this structure is formed, it is possible to form the pulsation absorbing device by a relatively simple method.
According to an eighth aspect of the invention, in the pump of any of the first to seventh aspects, the working fluid entrance side of the entrance passage and a working fluid entrance side of the exit passage are chamfered or rounded. When this structure is formed, since the fluid resistance at each fluid path is reduced, it is possible to increase the performance of the pump.
The working fluid entrance side refers to the side towards which the fluid flows in when the fluid is made to flow in the forward direction (load direction) as a result of operating the pump. The working fluid exit side is the side towards which the fluid flows out when the fluid is made to flow in the forward direction as a result of operating the pump.